[REG:6.1->6.2] Mdi child windows cannot be closed if any of them contain qtdatavisualization window containers

If any MDI child window contains qt data visualization windows Q3D*, none of the MDI children windows can be closed anymore (including ones that have nothing to do with Q3D* sub mdi windows).

Container windows themselve are not the issue, only if used with data visualization windows.

This commit is the likely culprit, as after adding Q3D* subwindow / container, the if is hit in QWidget::close() and the mdi sub window is no longer closed.

https://codereview.qt-project.org/c/qt/qtbase/+/368422